This invention relates to a lead frame and a resin-molded semiconductor device employing such a lead frame.
Metallic lead frames are often used for the assemblage of resin-molded semiconductor devices. The lead frame is formed in such a way that a thin metal plate is punched with a press or etched. FIG. 1 shows a prior-art lead frame formed by this method which is suitable for a semiconductor device of high packaging density. This lead frame is composed of a rectangular tab 2 for mounting a semiconductor element, tab leads 3 which support the tab 2, a plurality of leads 4 which extend to near the peripheral edge of the tab 2, a rectangular frame portion 5 which supports the leads 4 and the tab leads 3, and a dam piece 6 which joins the frame portion 5, the leads 4 and the tab leads 3 together and which prevents the outflow of a resin dissolving during the resin-molding operation. Further, guide holes 7 are provided at fixed intervals along both the side edges of the frame portion 5. For assemblage or transportation which employs the lead frame, the guide holes 7 are used as locating holes or as engaging and transferring holes.
In assembling the semiconductor device with such lead frame, the semiconductor element 1 is mounted on the tab 2, whereupon electrodes of the semiconductor element 1 and the inner ends of the leads 4 corresponding thereto are connected by wires 8. Thereafter, a region inside the dam piece 6 arranged on the rectangular frame portion 5 is molded with the resin so as to cover the semiconductor element 1 etc. with a molding portion 9. Subsequently, the dam piece 6 and the frame portion 5 are cut off to obtain the semiconductor device having flat leads. Also, in order to obtain deal-in-line type semiconductor devices, the leads 4 which protrude from the molding portion 9 are bent halfway.
In the semiconductor device utilizing the lead frame, problems to be stated below have been found by the inventors.
For example, the tab is low in strength because of the structure in which both its sides are supported by the two fine tab leads. Especially, in order to keep the size small in spite of recent trends to increase the number of leads, the widths of the leads (tab leads) cannot avoid becoming small. In order to narrow the leads (to widths of, for example, 0.3 mm), a material for forming the lead frame is made thinner from the viewpoint of punching. By way of example, the thickness of the lead frame is approximately 0.15 mm. As a result, the tab is easily inclined or lifted even by a slight external force applied to the leads, and this constitutes a hindrance to the assemblage. For example, the semiconductor element is mounted on the tab of the lead frame by rubbing by the use of a semiconductor element-adsorbing jig such as collet. For this reason, the tab swings due to oscillations of the rubbing, and the semiconductor element and the tab are imperfectly bonded. This results in the problem that the semiconductor element cracks when pushed by the flow of the resin during the resin-molding. Moreover, under the operating state of the semiconductor device, a thermal stress acts between a part in which the semiconductor element and the tab are perfectly bonded and a part in which they are imperfectly bonded, on account of heat generated from the semiconductor element. This also results in the problem of the appearance of cracks in the semiconductor element. Further, the resin is caused to flow in towards the semiconductor element 1 in the direction of arrow A indicated in FIG. 1. When, at this time, a corner 2a of the tab 2 is slanted downwards or upwards, the tab 2 inclines more in the slanting direction due to the flow of the resin. The tab 2 is also pushed in the direction of arrow B by the resin. This results in the problem of the breaking of the wire interposed between the bonding region of the semiconductor element and the lead or the problem of the contact between the wires. Further, air bubbles remain in or on the molding portion on account of the development of vortices ascribable to the inclination of the tab. This results in the problem that the moisture resisting property degrades or that the external appearance becomes inferior due to hollows.